1. Field of the Invention
The present invention generally relates to thermally stable abrasive compacts or tool blanks comprising a polycrystalline cubic boron nitride (CBN) or wurtzitic boron nitride (WBN) abrasive or cutting layer bonded to a support material such as a cemented carbide. More particularly, the present invention relates to a method for making such thermally stable tool blanks by positioning a metal barrier disc between the support material and a sinterable, catalyst-free layer of cubic boron nitride or wurtzitic boron nitride particles, and subjecting such arrangement to temperature and pressure conditions effective for forming a composite compact.
2. Prior Art
Polycrystalline cubic boron nitride tool blanks have found wide acceptance in the machining art, especially for the machining of ferrous alloys. U.S. Pat. No. 3,743,489 discloses one such tool blank in which a unified mass of greater than 70 volume percent CBN crystals are bonded to a larger sintered carbide for support. The mass of cubic boron nitride crystals includes a metallic phase containing aluminum atoms and atoms of at least one alloying element selected from the group consisting of nickel, cobalt, manganese, iron, vanadium and chromium. Tool blanks of this type are not considered to be thermally stable due to the presence of the aluminum alloy in the CBN layer. Because the aluminum alloy has a coefficient of thermal expansion which is significantly different from that of cubic boron nitride, at elevated temperatures, such as may be encountered in cutting or milling operations, the aluminum alloy expands causing the CBN layer to crack or fracture.
U.S. Pat. No. 3,767,371 describes polycrystalline CBN tool blanks comprising a unified mass of greater than 70 volume percent CBN crystals bonded to a metal-bonded carbide support mass. The carbide material is predominantly tungsten carbide, titanium carbide, tantalum carbide or mixtures thereof and the metal bonding material is selected from the group consisting of cobalt, nickel, iron and mixtures thereof. Tool blanks of this type are not thermally stable because metal bonding material migrates from the carbide support to the CBN layer where it serves as a cement material. Once again, thermal instability is due to the fact that the metal bonding material has a coefficient of thermal expansion which is significantly different than that of cubic boron nitride.
U.S. Pat. No. 4,063,909 relates to cemented tungsten carbide supported cubic boron nitride compacts having a continuous layer of high temperature braze metal less than 0.5 mm in thickness disposed between the CBN layer and the carbide support. Such abrasive bodies can be prepared in situ by subjecting the CBN abrasive particles, the braze metal and the cemented carbide support or carbide molding powder to temperatures and pressures in the cubic boron nitride stable region of the boron nitride phase diagram. Suitable braze metals include titanium, nickel, cobalt, iron, chromium, manganese, vanadium, molybdenum, tantalum, platinum and alloys containing one or more of these metals.
U.S. Pat. No. 4,224,380 pertains to thermally stable tool components comprising between 70 and 90 volume percent self-bonded CBN particles; a metallic phase infiltrated substantially uniformly throughout, said phase comprising between about 0.05 and 3 volume percent of the CBN layer and said phase being selected from cobalt, cobalt alloys, and alloys of aluminum with a metal selected from the group consisting of nickel, manganese, iron, vanadium and chromium; and a network of interconnected pores dispersed throughout. Although CBN tool blanks of this type are thermally stable, it would nevertheless be desirable to provide thermally stable CBN tool blanks without a network of interconnected pores.
U.S. Pat. No. 4,673,414 describes a method for making re-sintered polycrystalline cubic boron nitride compacts which comprises placing sintered boron-rich polycrystalline CBN particles in a high pressure/high temperature (HP/HT) apparatus and subjecting the boron-rich CBN particles to a pressure and a temperature adequate to re-sinter the CBN particles. Products of this type are normally made without a carbide support in order to provide a thermally stable tool blank. If a cemented carbide support is employed, metal bond material such as cobalt infiltrates into the CBN layer with the result that the CBN layer is no longer thermally stable. Of course, in the absence of a cemented carbide support, the tool blank will be entirely composed of CBN and, therefore, will be quite expensive. Moreover, in the absence of a support, the CBN layer will be more susceptible to fracturing.
U.S. Pat. No. 4,797,326 provides a process for making supported CBN compacts by separately forming a sintered CBN compact and a plastically deformable support, mating the CBN compact and the support, subjecting the mated CBN compact and support to elevated pressure and temperature conditions sufficient to plastically deform the support surface into attachment with the compact surface, and recovering the resulting supported compact. Although this method is capable of providing supported thermally stable CBN tool blanks, it would be desirable to provide such tool blanks in a process which does not require separate manufacture of the CBN compact and the support mass.